A buildup of urban areas along the coast that sweeps auto and human waste into the waters during rainfall.

And a continual runoff of phosphorous from farmlands.

In contrast, mining and a dwindling ice cover in the winter remain a threat to Lake Superior, said David Allan, the project's lead researcher and a professor of aquatic sciences at the University of Michigan's School of Natural Resources and Environment.

"There are many environmental stressors that affect an ecosystem, and the Great Lakes is a poster child for that," he said.

The study, published Monday in the scientific journal Proceedings of the National Academy of Sciences, charts out the "cumulative stress" on the lakes, which provide 1.5 million jobs and $62 billion in paychecks annually, according to a 2010 study. In addition, the lakes provide fun for swimmers, anglers, boaters and other outdoor enthusiasts.

For three years, a team of 22 scientists from across the U.S. and Canada collected, then layered, data and reports and satellite images — previously available only piecemeal — to compile a single map that charts humans' impact on the Great Lakes.

Additionally, 161 of North America's Great Lakes researchers and natural resources managers filled out a lengthy questionnaire that ranked what they felt were the most damaging threats.

“It's almost a death-by-a-thousand-cuts syndrome.”

Peter McIntyre, study co-author

"(But) none of the things we addressed had zero impact," said McIntyre, who began working on the study as a postdoctoral fellow at U-M.

The work underscores that a restoration effort focusing on a single, high-profile threat — trying to battle zebra mussels, for example — may have minimal effect unless other threats are addressed, too.

McIntyre likened the lakes' red zones to a sick patient with multiple organ failure: "It's not going to do a lot of good to have a heart transplant if their kidneys and liver aren't working."

Many of the reddest areas hug the shoreline of urban areas — little surprise given industry and man's physical destruction of the natural shoreline.

But another reason a wide swath of red runs along the southern Lake Erie shoreline beginning near the Pennsylvania-New York border, continuing westward through Ohio and curling northerly to the Detroit area: It's agriculture and the runoff of fertilizer.

Those are "some of the heaviest loads of phosphorous in North America," Allan said.

The map confirms that the more than $1 billion federal Great Lakes Restoration Initiative is correctly targeting the most acutely threatened regions, Allan and McIntyre said.

But the lesser-threatened areas of the Great Lakes — highlighted on the map as blue — offer a lesson, too, the authors said. In those areas, less-extensive, less-expensive restoration efforts can have a wide impact given the interconnectedness of the Lakes.

While most resources understandably are aimed at the most acutely threatened areas, "the point is that there are low-hanging fruit at both ends of the spectrum," McIntyre said.

Lakes Erie and Ontario, shown primarily in red on the map, face the most threats, according to a study published in the scientific journal Proceedings of the National Academy of Sciences. Find the full map in interactive form at greatlakesmapping.org.(Photo: Great Lakes Environmental Assessment and Mapping Project)